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topic 59271

Car restoration zinc plating gone bad



May 18, 2013

Q. I am restoring an old car and a couple of years ago had a large amount of parts zinc plated in both blue and gold zinc. At the time some of the parts came back with a black and/or white corrosion which I've since learned was most likely white rust. These parts and any other parts that weren't done with a satisfactorily bright appearance were re-done at the time at my request. I've now taken all parts out of storage (in a dry environment) to find:

- Some parts (possibly those that were re-done) have white and/or black rust
- Some parts that were bright and shiny have slightly dulled and have what are best described as 'water spots' (white rust again, I am guessing?)
- Parts that were originally galvanized or black oxided, that were replated in blue zinc, have now dulled, some have gone 'hazy' and on same parts there are traces of brown rust (possibly where they were wired up and left bare-metal)
- I spilled some oil on two gold zinc parts and cleaned with warm water. Within an hour afterwards, brown surface rust started appearing. The base metal was somewhat pitted prior to plating. It looked like the plating hadn't sealed the metal underneath.

I attempted to contact the plater but he is no longer trading. To make matters worse, I had many small spring parts plated and I didn't know about hydrogen embrittlement at the time nor was I advised about it, so now some of the parts are junk. I believe some/all parts were stripped with sandblasting, unsure about what acid pickling was used. Some/all parts were barrel plated (using electroplating, not mechanical), unsure if bulkier parts were wired up in racks or barreled. Some parts were even bent and damaged at the time, presumably due to the barrel process.

Given the current state of the parts, I now have serious concerns about the plater's abilities and the integrity of the plated parts. Excluding the springs, I still want to use the other parts but I am wondering :-

- Most of the parts that were plated were fasteners, heater box brackets/mechanisms, hood safety latches / locks, and other various under-hood and under-dash parts. No suspension parts. I suspect apart from the spring parts, that most/all others would have metal likely equivalent to SAE Grade 5 bolts or softer. Given my above concerns about the plater's abilities and processes, could these parts have suffered AT ALL from hydrogen embrittlement or other issues due to the processes used, if the plater has done any of the steps wrong? (i.e., acid pickled too long) Or is the metal's integrity forgiving, if it is softer like Grade 5 or less?

- The parts are 30~40 years old so pre-plating some were pitted/rusty, not smooth/shiny. Does this allow for hydrogen to get trapped more easily or the zinc not to seal properly? (no polishing or linishing was done)

- I have decided to paint some of the parts. Again, given my concerns about the quality/integrity, and the fact that some parts are suffering from white rust, is it safe to follow the normal processes of preparing zinc for painting and painting over it? Or is the zinc going to fail under the paint in the future and rust from the inside-out? [note: I have prepared and painted over 30+ year old cad and/or zinc plating before]

Basically what I want to know is, is there any possibility the metal's integrity has suffered even slightly and are the parts still safe to use? I want to re-plate some parts and paint over others.

Also I'd like to give some advice for any restorers out there considering electroplating... Keep in mind that acid will strip old zinc in hidden places where the zinc may not be able to reach when replating (except hot dip gal), leaving a part rusting from the inside out. This is another thing the plater never told me. :( Always ask technical questions, a plater should patiently and happily answer; if you have a bad gut feeling about a plater, trust it! )

Please help!

Byron Reynolds
Car restorer - BRISBANE, Queensland, Australia


May 23, 2013

A. It sounds like you have been dealing with an incompetent plater, who did an overall lousy job and you'll need to have the parts replated elsewhere. Ask around among others who restore old cars, and you'll learn of competent plating shops.

Your springs may well be hydrogen embrittled, and it's best to replace them with new. The other parts which might be hydrogen embrittled are high-strength steel parts. Ordinary stamped or machined steel parts which have not been heat treat hardened are probably okay.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina


June 2, 2013

Q. Thanks for the reply Jeffrey!

Excluding the springs, I don't think I'd have any high-strength parts, they're mainly just brackets/fasteners, i.e. brackets for power brake unit, heater linkages, hood lock/latch etc. and associated Grade 5 bolts, washers etc., I guess they would all be just mild steel? Some brackets appear to be made of thick sheet metal (about 1/8~3/16) but I don't think they'd be high-strength...? (and there are definitely NO suspension/brake parts)

In the worse case scenario, could the mild steel still be embrittled a little or suffer some other issue? Or would the worse I'd be looking at is they were left in the acid too long and very slightly eaten away? (doubt it as there were still spots of surface rust here and there the first time through!) I can deal with poor quality plating, but it's another story if the metal is ruined.

For parts I want to paint, would I be safe to clean up any white-rust and just paint over the zinc?

Just trying to work out how pedantic I should be about replacing parts / replating / painting! I'm fussy about quality and would hate to have to replace a part in the future due to some weakness or failure (the joy of car restoration!).

Byron Reynolds
- BRISBANE, Queensland, Australia


June 3, 2013

A. I'd replace all the springs. The non-suspension/brake parts you describe should not have a hydrogen embrittlement problem. Unlikely any parts were in an acid long enough to materially reduce any dimensions, but you should be able to tell by looking/measuring.

Sure you can paint over old zinc plating which has been wire brushed. Use a decent primer.

Good luck.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina

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